1. Field of the Invention
The present invention relates to a method of and an apparatus for stretching a thermoplastic polymer film monoaxially in a transverse direction or biaxially in longitudinal and transverse directions at the same time, if desired.
2. Prior Art
There have been proposed methods of stretching a thermoplastic polymer film in which the film is gripped along its opposite selvages between a pair of substantially coplanar pulleys rotating in opposite directions and a pair of endless belt means respectively trained arcuately around the pulleys, and then stretched transversely by moving the gripped film selvages, respectively, along a pair of divergent substantially semicircular arcuate paths formed, respectively, around the pulleys while it is heated. The film is reversed in direction of travel during movement of the gripped selvages along the arcuate paths. Such methods are described in copending U.S. patent application Ser. No. 133,146 filed Mar. 24, 1980 and Ser. No. 196,121 filed Oct. 10, 1980, respectively, which have been assigned to the present assignees. Such proposed methods suffer from the drawback that the distance of longitudinal movement of the film while being stretched varies or differs widthwise of the film between the selvages and the central web portion thereof. The maximum difference in distance (d) of movement is theoretically represented by the formula d.sub.max =r(.pi.-2) (where r is the radius of the pulley). However, departures in excess of the theoretical maximum value have been observed in practice because, under longitudinal contractive stresses created during lateral stretching of the film, the unsupported central web portion of the film tends to be distorted where the film is reversed in its direction of travel, i.e. the film material of that portion tends to take a short-cut by turning about a nearly chevron-shaped line at the place of reversal instead of turning about a tangential line common to the two pulleys on their farthermost sides. The stretched film is irregular in molecular orientation particularly in the longitudinal direction thereof and has varying strength in different directions.